Most process control schemes are closed-loop, depending upon a feedback signal that either directly or indirectly indicates the effect on the process output of changes in the output of one or more actuators. In an open-loop control process, a feedback signal is normally not available, typically because the output of the process is difficult, expensive, or impossible to monitor in a timely fashion. Open-loop processes are sometimes automatically controlled by using a model to predict the process output as a function of the output from the controlled actuator. The accuracy with which the process is controlled then depends simply on how accurately the model reflects the actual behavior of the process.
For example, a physician using an infusion device (actuator) to administer a drug intravenously to a patient may program an infusion device controller to achieve user-specified plasma drug concentrations based on a pharmacokinetic model of the drug being infused. The infusion device controller uses the pharmacokinetic model to dynamically determine the drug delivery rate at which the infusion device should be operated to achieve the specified plasma drug concentration. The infusion device infuses drug to the recipient, and the drug becomes present within the plasma of the recipient at some concentration. Ideally, the ability of the infusion device controller and the infusion device to achieve the specified plasma drug concentration would depend on the accuracy and applicability of the pharmacokinetic model. However, most drug infusion devices have a limited range of delivery rates, and a limited resolution for specifying or controlling the delivery rate. If the control method computes a delivery rate that is outside the available range of delivery rates and/or the actual delivery rate does not equal the computed rate due to lack of resolution of the infusion device, a significant difference between the theoretical and actual plasma drug concentrations may develop over time. Furthermore, if, for example, sensors within the infusion device detect an occlusion of the catheter line connecting the infusion device to the recipient and cause the infusion device to halt drug delivery, the theoretical and actual plasma drug concentrations may diverge unless the controller and pharmacokinetic simulation are advised of the interruption in drug delivery and compensate accordingly once drug delivery is resumed. Thus, in a model-based control method, the model simulation must be apprised of the actual output of the actuator (input to the process), not the desired output of the actuator.